The objective of this competitive renewal application is to develop a catheter-based diagnostic technique for localization and characterization of rupture-prone atherosclerotic plaques. We propose to combine into one unique, clinically compatible system (i) our recently developed time-resolved laser-induced fluorescence spectroscopy (TR- LIFS) technique, which provides a direct evaluation of plaque composition, with (ii) high-resolution intravascular ultrasonography (IVUS) which allows a visual reconstruction of plaque microanatomy. The resulting system is to enable detection and monitoring of both compositional and structural features of atherosclerotic lesions which are predictive of plaque rupture. In this application, we consider the current paradigm for understanding the progression of atherosclerotic disease, criteria for defining and detecting vulnerable plaques, and final steps in plaque rupture, to develop an apparatus and associated methodologies for detection of one or a combination of factors associated with plaque vulnerability. Over the past three years, with NIH funding, we have developed a TR-LIFS clinically compatible apparatus (instrumentation and software), tested/validated this apparatus in an in-vivo atherosclerotic animal model, and established the infrastructure and collaborations that allows study of atherosclerotic plaques in patients. Currently, we investigate carotid plaque in patients undergoing carotid endarterectomy using this technique. Our results demonstrate that TR-LIFS can predict accumulation of inflammatory cells in the.plaque fibrous cap, one key marker of plaque vulnerability. In this competitive renewal application we seek to demonstrate the feasibility of TR-LIFS technique for atherosclerotic plaque clinical characterization while developing synergy with high-frequency IVUS for guidance of TR-LIFS investigations and increased specificity of plaque diagnostic. Three specific aims will be addressed: (1) To establish the clinical feasibility of TR-LIFS for diagnosis of rupture- prone plaques by analyzing information from TR-LIFS measurements in patients undergoing carotid endarterectomy. (2) To develop an ultrasonic - guided TR-LIFS prototype that provides information for characterization of both plaque morphology and biochemistry/inflammation at the same location. (3) To design, build and test a diagnostic catheter combining TR-LIFS and ultrasonic (IVUS) detection. The probe will be tested initially on human arterial segments (ex- vivo) and then in an animal model (in-vivo). This study will facilitate direct translation of research findings on carotid plaques and catheter design into clinical practice for identifying and treating those patients who are more likely to have a high-risk plaque prior to any stroke symptoms. Also, the study will serve as paradigm for detection of vulnerable plaques in other arterial beds including coronary arteries.